1. Field of the Invention
The present invention relates to a field effect type thin film transistor having a plurality of gate electrodes arranged between source and drain regions.
2. Description of the Related Art
In a conventional field effect type transistor, if source and drain regions, both formed by diffusing impurities in a semiconductor layer at a high concentration, are brought into direct surface contact with a channel region, a depletion layer, occurred at a boundary surface between the drain and channel regions with an application of a high voltage, becomes so large thereby easily causing an avalanche phenomenon, in which electrons are excited and induce pairs of activated electrons and positive holes. Some of the high-energy hot electrons thus dashed into a gate insulation film, and are caught therein. Due to the behavior of the hot electrons, problems such as variations in threshold voltage and mutual conductance, or lowering of withstand voltage, occur.
As a counterplan to such problems, a structure of interposing a buffer region, in which impurities are diffused at a low concentration, between the drain and source regions of the semiconductor layer is well known. This structure is called as "Lightly Doped Drain" (LDD), and U.S. Pat. No. 4,697,333 discloses an example of the LDD structure. This prior art document discloses a construction for making a single N-MOS by using a monocrystalline semiconductor substrate.
In the meantime, recently, the development of a field effect transistor using a thin film semiconductor layer is actively conducted. The TFT of this type enables realization of new devices such as liquid crystal display panels and large capacity memory devices, which have not been realized with silicon semiconductor substrates. These TFTs require, in addition to a single-transistor element such as N-MOS type and P-MOS type, a circuit in which a plurality of gate electrodes are formed in one semiconductor island, such as an AND circuit, a NAND circuit, a OR circuit, a NOR circuit, and a clocked inverter circuit. In this case, since a thin film transistor is significantly different from a monocrystalline transistor in characteristics of a leak current, an on current, and a withstand voltage, the thin film transistor having, the same construction as that of the monocrystalline transistor is not considered to be putted to practical use. However, a construction of the above described circuit appropriate for a thin film transistor has not yet been examined.